1. Field of the Invention
This invention relates to an apparatus and method for testing or investigating particles present in a fluid using dielectrophoresis, for example to determine the dielectrophoretic characteristics, or to identify the presence and/or relative concentration of a particular type or types of particle in the fluid.
2. Description of the Related Art
Dielectrophoresis (DEP) is the translational motion of a particle caused by polarisation effects in a non-uniform electric field. Unlike electrophoresis, no overall electrical charge on the particle is necessary for DEP to occur. Instead, the phenomenon depends on the magnitude and temporal response of an electric dipole moment induced in the particle, and on the force produced as a consequence of the electric field gradient acting across the particle. The magnitude of the dielectrophoretic force F.sub.dep on a spherical particle of radius a is given by: ##EQU1##
where .di-elect cons..sub.m is the absolute permittivity of the suspending medium, .gradient.E signifies the gradient in the electric field, and .di-elect cons..sub.p * and .di-elect cons..sub.m * are complex permittivities of the particle and its surrounding medium, respectively. The complex permittivity is given by .di-elect cons.*=.di-elect cons.-j.sigma./.omega., where .di-elect cons. is the absolute permittivity, .sigma. is the electrical conductivity, .omega. is the angular frequency of the electric field and j=-1+L . The term Re indicates that the real part of the expression within the square brackets of equation (1) is to be taken.
For particles suspended in a uniform aqueous electrolyte, the permittivity and conductivity of the suspending medium usually remains approximately constant over the frequency range 100 Hz to 100 MHz, whereas for the particles themselves these parameters can vary significantly. The term (.di-elect cons..sub.p *-.di-elect cons..sub.m *) can therefore be positive or negative, and thus over an extended frequency range a particle can exhibit both positive DEP (movement towards areas of high field strength) and negative DEP (movement towards areas of low field strength).
Differences in the dielectrophoretic frequency response of particles can be used to selectively separate them by dielectrophoresis. An example of a particle separator which operates on this principle is described in International Patent Application WO-A1-9422583 in which a fluid containing two types of particles flows over electrodes producing a non-uniform electric field which is controlled so that the two types of particle experience different resultant forces and the fluid flow can remove one particle type preferentially. This separator can thus separate dielectrophoretically different particles or cells, but to be used effectively the dielectrophoretic behaviour of the two different particle types should be already known.
Pin-plate electrodes have been used for this purpose to determine the dielectrophoretic characteristics of particular particle types, but the procedures are laborious and time consuming.
Gascoyne et al Meas. Sci. Technol. 3 (1992) at pages 439 to 445, determines the DEP behaviour of 200-300 particles, specifically mammalian cells, by automatic image analysis. However, as with the use of pin-plate electrodes the DEP response of the cells can only be measured at a single frequency at a time. Because the frequency range of interest in DEP is relatively large (typically 100 Hz to above 10 MHz), and several data points per decade may be required, many single-frequency experiments need to be obtained for a sufficiently wide spectrum. Using this method it is therefore cumbersome and time-consuming to obtain a dielectrophoretic spectrum over a large frequency range, so as to facilitate the observation or manipulation of cells or particles.
There is therefore a need for a better way of determining the characteristics of particles in a fluid and/or of identifying one or more particle types present.